Hazel thanks for your note. (LOVE The Sartorialist!!). All black is often the default colour because it's the easier to unify (imagine an ensemble full of slightly different shades of red), it's easy on the eyes, and the audience tends to focus more on the music than if the ensemble were wearing different colours. The problem with saying go for it, wear any colour is what one person considers stylish another will consider completely inappropriate. There there's the issue of colour clashing. And in classical music, there are known stereotypes about what kind of dress is associated with certain types of music. Opera performers (and audiences) tend to go all out with style and flamboyance; Baroque performers (and audiences) tend not to consider fashion a big deal and are very low-key with their apparel. Ultimately, no, I don't think ensembles should be able to wear any colour; then it looks like a rehearsal.